This application claims benefit of Japanese Application No. 2000-350117 filed in Japan on Nov. 16, 2000, Japanese Application No. 2000-350118 filed in Japan on Nov. 16, 2000, Japanese Application No. 2000-350119 filed in Japan on Nov. 16, 2000, the contents of which are incorporated by this reference.
1. Field of the Invention
The present invention relates to a printer such as a sublimable type thermal printer having a freely attachable or detachable battery as a power supply for printing operation. More particularly, the present invention relates to a printer capable of correcting a current to be conducted to a thermal head for the purpose of stabilizing a printing density. The printer thus achieves printing with stable image quality ensured, offers improved performance, and contributes to realization of a low-cost compact design.
2. Description of the Related Art
In recent years, heat-sensitive heat-transfer printers capable of presenting high-definition images owing to the ability to print images full of colors have been widely adopted as devices that produce hardcopies of images received from a personal computer, a camera-built-in video tape recorder, an electronic still camera, or the like.
As far as a conventional heat-sensitive heat-transfer printer is concerned, print paper as well as an ink sheet is pressured against and sandwiched between a platen roller and a thermal head. The ink sheet is produced by applying a plurality of heat-sublimable color dyes to a base film, and positioned so that the heat-sublimable dyes will stick to the print paper. A plurality of heating elements are arranged on one side of the thermal head. When a current is conducted to the thermal head, the heating elements appropriately generate heat according to print data. Consequently, the heat-sublimable dyes are heated through the base film. This causes the heat-sublimable dyes to sublime. Eventually, the heat-sublimable dyes are transferred to the paper for the purpose of printing.
In the past, many proposals have been made of the foregoing types of printer and intended to improve printing quality and reduce costs. For example, Japanese Unexamined Patent Application Publication No. 5-238046 describes a battery-driven printer and Japanese Unexamined Patent Application Publication No. 7-195729 describes a heat-sensitive heat-transfer recording device.
The battery-driven printer described in the Japanese Unexamined Patent Application Publication No. 5-238046 has a rechargeable battery as a power supply that permits photo-printing. When photo-printing is not performed, a rectified dc voltage source composed of an ac rectifier, a switching unit, a voltage control unit, and a secondary rectifier, is used to charge the rechargeable battery. During photo-printing, the rechargeable battery discharges to supply power required to perform photo-printing. A voltage drop stemming from the discharge is detected. The voltage control unit controls the rectified dc voltage source so that the rectified dc voltage source will develop a dc voltage corresponding to the voltage drop. The voltage control unit then uses a charging/superposition switching unit and a superposition unit to superpose the output of the rectified dc voltage source on the output of the battery for correction. The voltage control unit thus stabilizes the power to be supplied from the battery to improve a quality of photo-printing.
Moreover, the heat-sensitive heat-transfer recording device described in the Japanese Unexamined Patent Application Publication No. 7-195729 performs thermal recording using a thermal head. An incorporated photo-printing voltage sensing unit detects a voltage applied to the thermal head, then, it develops a voltage to be applied to the thermal head based on an output voltage of a voltage adding circuit.
The voltage adding circuit adds up an output voltage of a peak-value detecting unit, which detects a peak value of an output voltage of the photo-printing voltage sensing unit, and a voltage set by a photo-printing voltage setting unit. In other words, the voltage applied to the thermal head is detected and then is controlled in order to stabilize the output voltage of the thermal head. Thus, a printing density is stabilized.
Furthermore, other proposals intended to improve printing quality include, for example, a proposal for a density/gray scale controlled printer described in Japanese Unexamined Patent Application Publication No. 6-91916 and a proposal for an image forming device described in Japanese Unexamined Application Publication No. 2000-135809.
The density/gray scale controlled printer described in the Japanese Unexamined Patent Application Publication No. 6-91916 conducts a current to heating resistance elements and records an image on print paper using the energy of heat generated from the heating resistance elements. Power is compensated based on the counted number of conducting elements. Thus, excellent color reproducibility is guaranteed on a constant basis all the time.
Moreover, the image forming device described in the Japanese Unexamined Application Publication No. 2000-135809 adopts a thermal head designed for a heat-transfer printer or a heat-sensitive printer in efforts to prevent occurrence of an uneven density among lines. Incidentally, the uneven density is attributable to the fact that the larger the number of heating resistance elements driven simultaneously among all heating resistance elements is, the smaller a current to be fed to each heating resistance element is. Herein, the number of all heating resistance elements is the same as the number of dots constituting one line. In the image forming device, calculation is performed using photo-print data representing one line. Consequently, a density can be corrected based on the pulse duration of current conduction pulses. Moreover, occurrence of an uneven density such as a white streak created between lines can be prevented.
Incidentally, there is a demand for printers that are inexpensive and able to produce high-definition prints. Moreover, printers that are compact, lightweight, and portable are strongly demanded so that printing can be performed at any time in any place.
In order to realize a portable printer capable of satisfying the above demands, the printer must be able to be driven using a battery alone. In this case, since the printer can be used even in places where an ac power supply is unavailable, its usefulness improves. When an emphasis is put on image printing, a heat-transfer printer such as the aforesaid sublimable printer is preferred because it offers high image quality.
However, the heat-transfer printer requires a large amount of power for heat-transfer printing. Moreover, since the capacity of a battery power supply is limited, occurrence of a voltage drop during use is unavoidable. As long as an ac power supply can be used, an applied voltage is held constant and unevenness in printing image quality is limited. However, when an attempt is made to use a battery alone to perform heat-transfer printing, a large obstacle must be overcome. Namely, a large voltage drop dependent on the degree of exhaustion of the battery must be coped with, and satisfactorily stable image quality must be attained.
In consideration of the above demands, the battery-driven printer described in the Japanese Unexamined Patent Application Publication No. 5-238046 is unacceptable, though it can maintain a predetermined photo-printing voltage all the time. Namely, the battery-driven printer has a drawback that it must be powered using an ac power supply but cannot be driven with a battery alone.
Moreover, the heat-sensitive heat-transfer recording device described in the Japanese Unexamined Patent Application Publication No. 7-195729 employs an assembly that detects a voltage applied to a thermal head, controls the applied voltage so that an output voltage of the thermal head will remain constant, and thus stabilizes a printing density. In addition, a peak-value detecting unit is employed for detecting the peak of a voltage so as to accurately measure the voltage applied to the thermal head. However, the heat-sensitive heat-transfer recording device is not oriented to be driven using a battery alone. The patent application publication does not describe a correction technology or the like intended to cope with a variation in an output voltage of a battery.
The conventional sublimable heat-transfer printers have a drawback that when a battery is adopted as a power supply, a large voltage drop dependent on the degree of exhaustion of the battery cannot be coped with, and printing cannot be achieved with sufficiently stable image quality ensured.
Moreover, in the thermal head employed in the heat-sublimable printer, a correlation expressed below is, generally, established relative to an amount of energy to be applied to the thermal head during photo-printing.
E=kV2 t/Rxe2x80x83xe2x80x83(1) 
where E denotes an amount of energy that permits printing to be achieved at a certain photo-printing density, and k denotes the heat efficiency of the head. Moreover, V denotes a voltage to be applied to the head, R denotes a resistance offered by the head, and t denotes a conduction time during which a current is conducted to the thermal head.
In the printer in which the relationship provided as the expression 1 is established, important factors are what is the amount of energy E ensuring a maximum density for photo-printing and what is the conduction time during which a current is conducted to the thermal head. The amount of energy E ensuring a maximum density is pre-set to a fixed value. On the other hand, when the conduction time t is extended, the head resistance R can be increased but a printing time itself gets longer. In the case of a color printer, a printing action is performed relative to four color inks of, for example, yellow (Y), magenta (M), cyan (C), and transparent of an overcoat (OP). The printing action is therefore performed four times. If the conduction time is extended, the printing time gets very long. This is unpractical and unacceptable. Conventionally, the conduction time is made short. Accordingly, the voltage to be applied to the thermal head ranges from 22 V to 28 V, and the head resistance ranges from about 7 kxcexa9 to about 10 kxcexa9. In a circuit including the thermal head, not only the thermal head offers a resistance but also the circuit itself offers a resistance. Therefore, when the applied voltage and head resistance are made high, a power loss is minimized. As long as power is supplied sufficiently, it is advantageous that the voltage and resistance are set as mentioned above.
However, when it comes to a portable printer, a very heavy battery cannot be adopted in terms of portability. In order to realize a supply voltage of 24 V, twenty nickel-hydride secondary batteries must be connected in series with one another. This contradicts the concept of a portable printer. Consequently, when a battery is selected in consideration of portability, the supply voltage ranges from about 7.2 V to about 7.6 V.
Using a dc-to-dc converter, the supply voltage is boosted to range from 22 V to 28 V as it conventionally is. The power loss caused for the aforesaid reason is overcome. However, the dc-to-dc converter causes an enormous power loss. This discourages realization of a power supply optimal to a mobile printer. When consideration is taken into an enormous power loss caused by the dc-to-dc converter, a large space occupied thereof, the weight thereof, and heat dissipated thereby, a portable printer should be designed so that a supply voltage will be applied to a thermal head as it is. Accordingly, a resistance to be offered by the thermal head is set to a proper value. This conceivably results in a heat-sublimable printer capable of satisfying the demand for portability to the maximum. In particular, a correcting means for performing correction such as printing ratio correction so as to stabilize a printing density of inks transferred from the thermal head helps satisfy such a demand for portability to the maximum.
FIG. 15 is a circuit diagram schematically showing the circuitry in accordance with a related art of a printer having an ordinary power supply. FIG. 13 and FIG. 14 concerned with an embodiment of the present invention will also be referred to in order to describe underlying problems of the printer in accordance with the related art.
The basic circuitry for a heat-sublimable printer is schematically shown in, for example, FIG. 13. Specifically, the printer has the circuitry composed of a power supply E, a resistor Rc, and a resistor Rh. The resistor Rc is a circuit element that offers a resistance. The resistor Rh offers a resistance corresponding to a resistance offered by a plurality of heating resistance elements incorporated in a thermal head. Incidentally, the plurality of heating resistance elements incorporated in the thermal head are provided the same number as the number of dots to be created, for example, 960.
When the above printer is designed to satisfy the requirements for a portable printer, the supply voltage E is, as mentioned above, set to 7.6 V, the resistance Rc is set to 1 xcexa9, and the resistance Rh corresponding to a resistance offered by the heating elements incorporated in the thermal head is set to 750 xcexa9. At this time, the number of heating resistance elements or heads that are turned on to create dots shall be N and a current flowing into the thermal head shall be i.
In the printer designed to satisfy the requirements for a portable printer, for example, when all the heating elements responsible for 960 dots are turned on, the current i flowing into each heating element is expressed as follows.                     i        =                                            7.6                              Rc                +                                  Rh                  N                                                      xc3x97                          1              N                                =                                                    7.6                                  1                  +                                      750                    960                                                              xc3x97                              1                960                            xc3x97              1000                        =                          4.444              ⁢                              (                mA                )                                                                        (        2        )            
When only one heating element of the thermal head is turned on to create one dot, the current i is expressed as follows.                     i        =                                            7.6                              Rc                +                                  Rh                  N                                                      xc3x97                          1              N                                =                                                    7.6                                  1                  +                  750                                            xc3x97                              1                1                            xc3x97              1000                        =                          10.120              ⁢                              (                mA                )                                                                        (        3        )            
In order to stabilize a printing density of inks transferred from the thermal head, it is necessary to detect how many heads (heating resistance elements) incorporated in the thermal head are turned on. A voltage to be applied must then be regulated based on the result of the detection. Since the supply voltage E is fixed, voltage regulation is impossible to do. Consequently, even if a total resistance varies depending on how many heating resistance elements are turned on or off, the current flowing into each heating resistance element must be held constant. This makes it necessary to perform printing ratio correction.
Specifically, the printing ratio correction is intended to reliably reproduce a printing density. Now, when a minimum current flows (a current flows into each of 960 heating resistance elements because the elements are turned on), a correction value shall be 100%. When the number of heating resistance elements that are turned on is 1, the correction value is calculated from the numerical values provided by the expressions (2) and (3) as 4.444/10.120=43.9%. FIG. 14 graphically shows the correction coefficient for a printing ratio in relation to the number of heating resistance elements that are turned on. In other words, unless the current is reduced by up to 56% in proportion to the number of heating resistance elements that are turned on to create dots, energy to be applied to each element cannot be held constant. Consequently, the correction ranges widely.
In a printer employing an ordinary power supply, the supply voltage developed from the ordinary power supply shall be 24 V, the resistance Rc shall be 1 xcexa9, and the resistance R corresponding to a resistance offered by all the heating elements of a thermal head shall be 7000 xcexa9. In this case, for example, when all the heating elements responsible for 960 dots are turned on, the current i flowing into each heating element is expressed as follows:                     i        =                                            24                              1                +                                  7000                  960                                                      xc3x97                          1              960                        xc3x97            1000                    ≈                      3.015            ⁢                          (              mA              )                                                          (        4        )            
When only one heating element is turned on to create one dot, the current i is expressed as follows:                     i        =                                            24                              1                +                7000                                      xc3x97                          1              1                        xc3x97            1000                    ≈                      3.428            ⁢                          (              mA              )                                                          (        5        )            
In this case, printing ratio correction is intended to reliably reduce a printing density. Now, when a minimum current flows (a current flows into each of 960 heating elements because the 960 heating elements are turned on), a correction value shall be 100%. When the number of heating elements that are turned on is 1, the correction value is calculated from the numerical values provided by the expressions 4 and 5 as 3.015/3.428=approx. 88.0%. In other words, as far as the printer employing an ordinary power supply is concerned, the correction ranges narrowly. Providing the printing ratio correction is not performed, only a difference in an amount of energy that can be corrected with the correction value set to 12% at most would take place.
Assume that in consideration of portability, a printer is designed to adopt 7.6 V and 750 xcexa9 as a supply voltage and a resistance offered by a thermal head respectively as mentioned above. In this case, the correction value assumes the aforesaid values. The range of the values of the correction is much wider than that required in a printer that is designed to adopt 24 V as a supply voltage provided by an ordinary power supply and 7000 xcexa9 as a resistance offered by a thermal head. When the correction ranges widely, how to control heat remaining in the thermal head after the thermal head is driven in order to cause the heating elements incorporated in the thermal head to generate heat becomes a big obstacle that must be overcome.
In order to overcome the obstacle, a printer may be designed to adopt 24 V as a supply voltage and 7000 xcexa9 as a resistance offered by a thermal head, because the correction ranges narrowly. In this case, it is inferred how much heat remains in each heating resistance element when photoprinting a gray-scale level. Thus, a current flowing into each heating resistance element may be controlled. However, as mentioned above, when a printer is designed to adopt 7.6 V as a supply voltage and 750 xcexa9 as a resistance offered by a thermal head, the correction ranges widely. In this case, it is impossible to achieve high-precision correction by inferring heat remaining in each heating resistance element and thus controlling a current that flows into each heating resistance element.
In the density/gray-scale controlled printer described in the Japanese Unexamined Patent Application Publication No. 6-91916, power is compensated based on the counted number of elements to which a current is conducted. This results in excellent color reproducibility that is stable all the time. However, the power compensation implemented in the printer is such that: the number of heating resistance elements to which a current is conducted is counted; an average resistance offered by the thermal head is corrected by referencing the data representing the number of elements to which a current is conducted and being stored in a RAM; and power to be fed to the heating resistance elements is controlled based on the corrected resistance offered by the thermal head. This poses a problem in that more sophisticated compensation cannot be expected.
In the image forming device described in the Japanese Unexamined Patent Application Publication No. 2000-135809, the technology for preventing an uneven density from occurring among lines is implemented. The uneven density is attributable to the fact that the larger the number of heating resistance elements, which are driven simultaneously, out of all the heating resistance elements that number the same as dots constituting one line, the smaller a current to be fed to each heating resistance element. Calculation is also performed using photo-printing data that represents one line. However, the patent application publication does not describe anything about correction adaptable to a printer designed to employ a battery power supply that is be preferred for portable use. The correction technology disclosed in the patent application publication has a drawback that it cannot be satisfactorily adapted to a portable printer.
Furthermore, when a printer is designed to employ a battery power supply suitable for portable use and adopt 7.6 V as a supply voltage and 750 xcexa9 as a resistance offered by a thermal head, the correction ranges widely as mentioned above. In this case, correction cannot be achieved highly precisely by inferring heat remaining in each heating resistance element and thus controlling a current that flows into each heating resistance element.
An object of the present invention is to provide a printer in which correction is controlled so that a printing density of inks transferred from a thermal head will remain constant despite a large voltage drop dependent on the degree of exhaustion of a battery. Consequently, the printer can achieve printing with sufficiently stable image quality ensured, and can be designed to offer improved performance and to be low-cost and compact.
Another object of the present invention is to provide a printer capable of achieving correction highly precisely despite adoption of a battery power supply suitable for portable use even with a structure in which a correction of current cannot help ranging widely. Moreover, the printer can be designed to offer improved performance and to be low-cost and compact.
Briefly, according to the present invention, there is provided a printer consisting mainly of a thermal head, a battery power supply means, a voltage detecting means, and a control means. The thermal head transfers a plurality of color inks successively to paper so that a color image can be printed on the paper according to image data. The voltage detecting means detects a voltage developed from the battery power supply means. The control means feeds power supplied from the battery power supply means to a load at the timing immediately preceding the transfer of the color inks to the paper. The control means instructs the voltage detecting means to detect a voltage developed from the battery power supply means immediately succeeding the feeding of power. The control means performs correction according to the result of the detection so that a printing density of inks transferred from the thermal head will remain constant irrespective of whether the voltage developed from the battery power supply means is high or low.
Moreover, according to the present invention, there is provided a printer consisting mainly of a thermal head, a battery power supply means, a battery detecting means, a display means, a voltage detecting means, and a control means. The thermal head transfers a plurality of color inks successively to paper so that a color image will be printed on the paper according to image data. The battery detecting means detects a remaining battery capacity of the battery power supply means by conducting a current to a first load. The display means displays an indication of the fact that the remaining battery capacity detected by the battery detecting means is judged to be equal to or smaller than a battery capacity required to perform printing. The voltage detecting means detects a voltage developed from the battery power supply means. The control means feeds power supplied from the battery power supply means to a second load, which is smaller than the first load, at the timing immediately preceding the transfer of the color inks to paper. The control means instructs the voltage detecting means to detect the voltage value developed from the battery power supply means at the timing immediately succeeding the feeding of power. The control means then performs correction according to the result of the detection so that a printing density of inks transferred from the thermal head will remain constant irrespective of whether the voltage developed from the battery power supply means is high or low.
Furthermore, according to the present invention, there is provided a printer consisting mainly of a thermal head, a battery power supply means, a voltage detecting means, and a control means. The thermal head transfers a plurality of color inks successively to paper so that a color image will be printed on the paper according to image data. The voltage detecting means detects a voltage value provided by the battery power supply means. The control means then feeds power supplied from the battery power supply means to a load at the timing immediately preceding the transfer of the color inks to the paper. The control means then instructs the voltage detecting means to detect the voltage value provided by the battery power supply means at the predetermined timing immediately succeeding the feeding of power. The control means then performs correction according to the result of the detection so that a printing density of inks transferred from the thermal head will remain constant irrespective of whether the voltage developed from the battery power supply means is high or low. Herein, in the correction performed by the control means, a correction value is determined based on the voltage detected by the voltage detecting means. When the same voltage is detected with transfer of each color ink, the correction value is determined to assume the same value.
According to the present invention, there is provided a printer consisting mainly of a thermal head, a battery power supply means, a voltage detecting means, and a control means. The thermal head transfers a plurality of color inks successively to paper so that a color image will be printed on the paper according to image data. The voltage detecting means detects a voltage value provided by the battery power supply means. The control means feeds power supplied from the battery power supply means to a load at the timing immediately preceding the transfer of the color inks to the paper. The control means then instructs the voltage detecting means to detect the voltage value provided by the battery power supply means at the predetermined timing immediately succeeding the feeding of power. The control means then performs correction according to the result of the detection so that a printing density of inks transferred from the thermal head will remain constant irrespective of whether the voltage developed from the battery power supply means is high or low. Herein, owing to the correction performed by the control means, when the voltage detected by the voltage detecting means is a first voltage value, printing is achieved at a maximum density.
In addition, according to the present invention, there is provided a printer consisting mainly of a thermal head, a first correction value determining means, a second correction value determining means, and a control means. The thermal head includes a plurality of heating elements that are used to print a color image on paper according to image data. The first correction value determining means calculates a printing ratio relative to a gray-scale level specified in image data representing one line out of the image data, and determines a correction value according to calculated printing ratios. The second correction value determining means performs an arithmetic operation using gray-scale data items based on which the heating elements generate heat so as to print one line according to the image data, and determines a correction value according to the result of the arithmetic operation. The control means then controls the amounts of heat dissipated from the heating elements according to the correction values determined by the first and second correction value determining means.
The above and other objects, features and advantages of the invention will become more clearly understood from the following description referring to the accompanying drawings.